TMR is a technique, known in the art, for creating channels in ischemic heart tissue, typically in the left ventricular wall of the heart, to improve the blood supply to ischemic myocardium. The technique is described, for example, by Mirhoseini, et al., in an article entitled "Transmyocardial Laser Revascularization: A Review," in the Journal of Clinical Laser Medicine & Surgery, vol. 11 (1993), pages 15-19, and by Bonn, in an article entitled "High-power lasers help the ischaemic heart," in The Lancet, vol. 348 (1996), page 118, which are incorporated herein by reference.
In TMR, as is known in the art, a computer-controlled laser is used to drill holes about 1 mm in diameter in the myocardium, communicating with the left ventricle, at a typical density of about one hole per square centimeter. Typically, the laser beam is delivered to the epicardium through an incision in the chest and the pericardium that exposes the beating heart. The laser, typically a CO.sub.2 laser or, alternatively, an excimer or Ho:YAG laser, fires pulses of about 1000 W, which photovaporize the myocardium and create channels through the endocardium into the ventricle. Blood at the outer, epicardial openings of the channels typically clots after a few minutes, but the inner portions of the channels, communicating with the ventricle, remain patent. It is hypothesized that during systole, blood flows through these channels into naturally-existing myocardial sinusoids, supplementing the impaired arterial blood supply.
Particularly when a CO.sub.2 laser is used, the laser is generally synchronized to the patient's ECG, so as to fire its pulse during systole, in the refractory period of the heart cycle. Firing the laser pulse at other points in the heart cycle can cause undesirable arrhythmias. The heart rate, myocardial thickness and other factors are used to determine the optimum energy level for each laser pulse.
U.S. Pat. Nos. 5,380,316 and 5,554,152, to Aita, et al., which are incorporated herein by reference, describe methods for intra-operative myocardial revascularization using an elongated, flexible lasing apparatus, which is inserted into the chest cavity of the patient. The distal end of the apparatus is directed to an area of the exterior wall of the heart adjacent to a ventricle and irradiates the wall with laser energy to form a channel through the myocardium.
U.S. Pat. No. 5,389,096, to Aita, et al., which is also incorporated herein by reference, describes methods and apparatus for percutaneous myocardial revascularization (PMR). A deflectable, elongated lasing apparatus is guided to an area within the patient's heart, and the distal end of the apparatus is directed to an area of interest in the inner wall of the heart. The wall is irradiated with laser energy to form channels therein, preferably without perforating the epicardium.
Since in PMR the channels are drilled from the inside of the heart outwards, there is no need for the channels to penetrate all the way through the heart wall, unlike more common TMR methods, in which the channels are drilled from the outside in. In other respects, however, the effects of PMR on the heart are substantially similar to those of TMR. Therefore, in the context of the present patent application, the term TMR will be used to refer to both extracardial and intracardial methods of laser revascularization of the myocardium.